The technical field generally relates to internal combustion engine aftertreatment systems. Many current powertrain systems include an aftertreatment system in the exhaust of internal combustion engines to meet emissions regulations or to reduce emissions of undesirable exhaust gas constituents. Aftertreatment systems often include multiple components, including particulate filters, oxidation catalysts, NOx adsorbers, NOx reduction catalysts, three-way catalysts, four-way catalysts, and can further include multiple components of the same type at various locations along the aftertreatment system flowpath. One well known way of removing oxides of nitrogen (NOx) from engine exhaust is Selective Catalyst Reduction (SCR). In this system a catalyst is used to facilitate a reaction between NOx molecules and a reductant to convert the NOx into common atmospheric gasses. One type of reductant that can be used in these systems is ammonia, which can be delivered by injecting an aqueous solution of urea into the exhaust stream in a component commonly referred to as a decomposition pipe. The decomposition pipe typically includes a mixing device to improve the uniformity of the reductant dispersion in the exhaust gas and a gas flow passage of some length to provide for residence time of the reductant in the exhaust gas. When an aqueous solution of urea is used as the reductant, heat from the exhaust gas evaporates water from the urea and provides the activation energy needed to chemically decompose the urea. Once heated to a sufficient temperature for a sufficient duration of time, urea is completely converted to ammonia and gaseous ammonia precursors.
A range of temperatures exists where urea decomposition will start; however, some of the urea will not completely decompose to gaseous products, but rather will only partially decompose, which results in solid products. If this partial decomposition happens to urea when it is present on a solid surface, such as a mixer element or decomposition pipe wall, the solid decomposition products can adhere to the surface, creating a deposit. Additionally, a different range of temperatures exists that are warm enough to cause water from the urea to evaporate, but where the temperature is insufficient to start rapid decomposition of urea. This condition can also result in a deposit of liquid or solid urea.
These deposits can cause several issues such as reduced SCR efficiency due to poor reductant distribution, increased exhaust restriction and eventually excess exhaust concentration of ammonia due to deposit break down when exhaust temperature increases.
Therefore, further improvements in this area of technology are desirable.